This invention relates to a method of removing detrimental metal ions from a hydrofluoric acid solution used in the manufacturing of semiconductor devices to clean silicon surfaces such as silicon wafer surfaces, the method using silicon granules as adsorbent.
In the manufacturing of integrated circuits including LSIs and VLSIs, an indispensable operation is the cleaning of the surfaces of silicon wafers. With the enlargement of the scale of integration in VLSIs the requirements for the cleanness of silicon wafers have become still severer.
In the cleaning of silicon wafer surfaces the primary purpose is to remove oxide film together with every contaminant, and it is usual to use a hydrofluoric acid solution to remove oxide film by etching. If the hydrofluoric acid solution contains ions of metals such as Cu and Au which are lower in ionization tendency than Si, the metal ions adhere to the cleaned wafer surfaces and affect the electrical characteristics of the semiconductor devices using the wafers. Therefore, it is essential to remove such detrimental metal ions from the hydrofluoric acid solution before the wafer cleaning operation. Although sufficiently refined hydrofluoric acid solutions are available now, often it is necessary to perform a solution refining operation just before the use in oreder to remove contaminants intruded into the acid solutions from containers, piping, etc. Moreover, hydrofluoric acid solutions used for cleaning silicon wafers need to be refined and reused. Since the used acid solutions contain detrimental metal ions which were carried by the silicon wafers, it is necessary to remove the metal ions from the used acid solutions.
It is possible to remove metal ions from an acid solution by a distillation method, but this method is unsuitable for mixed solutions such as hydrofluoric acid solutions used for cleaning silicon wafers because of changes in the chemical composition of the distilled solution. A conventional method is the use of an ion exchange resin. However, in this method the dissolution of organic matter from the ion exchange resin into the treated hydrofluoric acid solution offers a problem. Besides, the ion exchange method is applicable only to dilute hydrofluoric acid solutions: this method is not applicable to solutions having high hydrofluoric acid concentrations (e.g. 20-30%) or solutions containing ammonium fluoride together with hydrofluoric acid.
A recently developed method for the removal of detrimental metal ions represented by Cu ion from a hydrofluoric acid solution used to clean silicon wafers is an adsorption method using silicon granules as adsorbent, as shown in JP 3-102827 A. In the acid solution the metal Si surface of each silicon granule is exposed since oxide film is removed by the etching action of the acid solution. If the acid solution contains Cu ion which is an example of ions of metals lower in ionization tendency than Si, the reduction of Cu ion to metal Cu and the oxidation of metal Si to Si ion take place on the surface of each silicon granule, and the precipitated metal Cu deposits on the silicon granule surface. Therefore, Cu ion is removed from the acid solution. Also Ag ion and Au ion can be removed from the acid solution by the same process. This adsorption method is effective only for the removal of ions of metals lower in ionization tendency than Si, but in the practice of the cleaning of silicon wafers this limitation offers little problem since actual contaminants for silicon wafers in the cleaning solution are the metal ions that can be removed by the adsorption method.
However, by the above described adsorption method the removal of detrimental metal ions from a hydrofluoric acid solution takes a considerably long time because the rate of adsorption is low. In the case of continuously refining a hydrofluoric acid solution by passing the acid solution through a filter unit containing silicon granules as described in the aforementioned JP 3-102827 A, it is impracticable to desirably raise the flow rate of the acid solution because it results in a failure to sufficiently reduce the concentrations of detrimental metal ions in the outflow of the acid solution. Furthermore, by this adsorption method it is very difficult to reduce the concentration of every detrimental metal ion in a hydrofluoric acid solution used to clean silicon wafers to the recently required level which is below 0.1 ppb.